Spinal disc herniation, a common ailment, often induces pain, as well as neurologically and physiologically debilitating processes for which relief becomes paramount. If conservative treatments fail, the more drastic measures of discectomies and spinal fusion may be indicated. The latter treatment, while providing short term relief, often leads to excessive forces on facet joints adjacent to the fusion and creates further problems over time. Drastic treatments are usually unable to restore normal disc function. The loss of disc function has led to a number of disc prostheses that attempt to provide natural motion.
The literature documents that the Instantaneous Axis of Rotation (IAR) during sagittal rotation of the superior vertebra with respect to the inferior vertebra of a Functional Spinal Unit (FSU) in the cervical spine moves significant distances during flexion and extension of the spine (Mameren H. van, Sanches H., Beursgens J., Drukker, J., “Cervical Spine Motion in the Sagittal Plane II: Position of Segmental Averaged Instantaneous Centers of Rotation-A Cineradiographic Study”, Spine 1992, Vol. 17, No. 5, pp. 467-474). This motion varies widely between functional spinal units on an individual spine and between individuals and depends on age, time-of-day, and the general health and condition of the intervertebral discs, facet joints and other components of the FSU and spine. A moving IAR means that the superior vertebra both rotates and translates while moving with respect to the inferior vertebra of an FSU. Natural spinal motions place severe requirements on the design of a prosthetic disc; simple rotational joints are not able meet those requirements.
In addition, motion coupling between axial and lateral bending and other functional spinal units involved in the overall spinal motion increases the complexity and difficulty in developing a prosthetic disc replacement that realizes natural spinal motion. The complex facet surfaces in an FSU significantly influence and constrain sagittal, lateral and axial motions. The orientation of these facet surfaces vary with FSU location in the spine and induce wide variations in motion parameters and constraints. The complex motion of a superior vertebra with respect to the associated inferior vertebra of an FSU, certainly in the cervical spine, cannot be realized by a simple rotation or simple translation, or even a combination of rotation and translation along a fixed axis, and still maintain the integrity and stability of the FSU and facet joints.
One advantage of a general motion spatial mechanism of a disc prosthesis, as described in this application, is that it solves the natural motion problem for disc prostheses and offers a scalable mechanism for disc replacement without loss of general motion capabilities in the FSU.
Researchers have attempted to design a successful intervertebral disc for years. Salib et al., U.S. Pat. No. 5,258,031; Marnay, U.S. Pat. No. 5,314,477; Boyd et al., U.S. Pat. No. 5,425,773; Yuan et al., U.S. Pat. No. 5,676,701; and Larsen et al., U.S. Pat. No. 5,782,832 all use ball-and-socket arrangements fixed to the superior and inferior plates rigidly attached to the vertebrae of an FSU. However, these designs limit motion to rotation only about the socket when the two plates are in contact. As the literature points out (Bogduk N. and Mercer S., “Biomechanics of the cervical spine. I: Normal kinematics”, Clinical Biomechanics, Elsevier, 15(2000) 633-648; and Mameren H. van, Sanches H., Beursgens J., Drukker, J., “Cervical Spine Motion in the Sagittal Plane II: Position of Segmental Averaged Instantaneous Centers of Rotation-A Cineradiographic Study”, Spine 1992, Vol. 17, No. 5, pp. 467-474), this restricted motion does not correspond to the natural motion of the vertebrae, for either sagittal plane motion, or for combined sagittal, lateral and axial motion. Further, when the two plates, as described in the cited patents, are not in contact, the devices are unable to provide stability to the intervertebral interface, which can allow free motion and lead to disc related spondylolisthesis, FSU instability, and excessive facet loading.
As a further elaboration on the many ball-and-socket configurations, consider Salib et. al. (U.S. Pat. No. 5,258,031) as an example of previous efforts to address this problem. The Salib ball-and-socket arrangement only provides 3 independent axes of rotation and no translation when engaged.
During complex motions of an FSU, the superior vertebra, in general, requires translation along three independent directions. A sliding ovate structure in an oversized socket cannot perform such general translation motions, either, as it must engage in a trajectory dictated by its socket's geometrical surface and does not change the deleterious effects that may occur on the facet joints of the unit.
Currently known devices appear to have similar motion and instability limitations, such as the rocker arm device disclosed by Cauthen (U.S. Pat. Nos. 6,019,792; 6,179,874; 7,270,681), the freely moving sliding disc cores found in the Bryan et al. patents (U.S. Pat. Nos. 5,674,296; 5,865,846; 6,001,130; and 6,156,067) and the SB Charité™ prosthesis, as described by Búttner-Jantz K., Hochschuler S. H., McAfee P. C. (Eds), The Artificial Disc, ISBN 3-540-41779-6 Springer-Verlag, Berlin Heidelberg New York, 2003; and U.S. Pat. No. 5,401,269; and Buettner-Jantz et al. U.S. Pat. No. 4,759,766). In addition, the sliding disc core devices of the Bryan et al. and SB Charité™ devices do not permit natural motion of the joint for any fixed shape of the core.
With the above-described prosthetic devices, when the FSU extends, the prosthesis's sliding core, in some cases, generates unnatural constraining forces on the FSU by restricting closure of the posterior intervertebral gap in the FSU. Further, the core does not mechanically link the upper and lower plates of the prosthesis and is unable to maintain the intervertebral gap throughout the range of motion. Such conditions can contribute to prosthetic disc spondylolisthesis. In general, unconstrained or over-constrained relative motion between the two vertebral plates in a prosthetic disc can contribute to FSU instability over time.
Static loading in current prosthetic disc technology appears to be minimal and limited to mostly rigid support. For example, load bearing and shock absorption in the SB Charité™ design and others (e.g. Bryan et al., U.S. Pat. No. 5,865,846) rely on the mechanical properties of the resilient, ultra-high-molecular-weight polyethylene core to provide both strength and static and dynamic loading. The rigidity of the sliding core appears to offer little energy absorption and flexibility to meet the intervertebral gap requirements during motion, and may likely generate excessive reaction forces on the spine during flexion, forces that can potentially produce extra stress on facet joints and effect mobility.
More recent attempts to provide dynamic and static loading capability is taught in the series of patents by Ralph et al (U.S. Pat. Nos. 6,645,249, 6,863,688, 6,863,688, 7,014,658, 7,048,763, 7,122,055, 7,208,014, 7,261,739, 7,270,680, 7,314,487) wherein the force restoring mechanism begins with a multi-pronged domed spring between two plates and ends with a wave-washer as the force restoring element. The multi-pronged domed spring employs a ball-and-socket arrangement on the upper plate and allows relative rotations between the spring-lower plate and the upper plate. This arrangement, during normal FSU operation, places moments of force on the spring that tend to distort the spring and place high stresses on the set screws holding the spring down. The effects of force moments on the prongs and the dome spring is mitigated by later designs where various modifications of the spring element, as for example the spiral Belleville washer in U.S. Pat. No. 7,270,680, provides the spring more resilience to moments of force. As taught in these patents, the motion of the upper plate is limited to compression and rotation. Lateral and sagittal translations are not accommodated and so general motion in the FSU is not enabled by the device.
The work of Errico et al (U.S. Pat. Nos. 6,989,032, 7,022,139, 7,044,969, 7,163,559, 7,186,268, 7,223,290, and 7,258,699) elaborates on the mechanical design of the patents of Ralph et al. A specially designed Belleville type washer provides a restoring force to compressions. Rotations of the superior plate of the device in a fixed ball-and-socket arrangement transfers moments of force about the washer central axis to a rigid structure. It is notable that the instruction in these designs specifically proscribes lateral motions (sagittal and lateral translation). Errico et al. employ a tapered projection attached to the ball to limit rotation angles.
Another approach to incorporate dynamic and static force response is taught by Gauchet (U.S. Pat. Nos. 6,395,032, 6,527,804, 6,579,320, 6,582,466, 6,582,468, and 6,733,532) wherein a hydraulic system provides shock absorption by means of a cushion between two plates contained within sealed flexible titanium bellows. Gauchet suggests the bellows can be designed to accommodate lateral forces and axial rotation that is permitted by the cushion, which, to allow sliding motion, is not attached to at least one plate. The titanium bellows can accommodate some axial rotations, but do not seem suitable for other rotations, which can cause excessive stresses on the bellows. A cushion internal to the cylinder, being flexible and not attached to at least one plate, can accommodate any rotation (U.S. Pat No. 6,582,466 and 6,733,532).
Fleishman et al in U.S. Pat. Nos. 6,375,682 and 6,981,989 utilize hydraulic action coupled with a flexible bellows to mitigate sudden forces. The bellows concept is similar to that of Gauchet.
Eberlein et al (U.S. Pat. No. 6,626,943) utilizes a fiber ring to enclose a flexible element. The forces and moments of force are absorbed by the ring and the flexible element. The device taught in this invention uses a boot in much the same manner as Eberlein's fiber ring. Other inventions teach this concept as well, namely, Casutt in U.S. Pat. No. 6,645,248. Diaz et al (U.S. Pat. No. 7,195,644) also uses a membrane and enclosed cushioning material in their ball and dual socket joint design.
Middleton suggests a variety of machined springs as the central component of a disc prosthesis in U.S. Pat Nos. 6,136,031, 6,296,664, 6,315,797, and 6,656,224. The spring is notched to allow static and dynamic response primarily in the axial direction of the spring. But, lateral and sagittal translations and general rotations appear to be problematic in these designs. The ability of such springs to tolerate off-axis compression forces may also be problematic.
Gordon instructs deforming a machined spring as the principle separating and force management component (U.S. Pat. Nos. 6,579,321, 6,964,686, and 7,331,994). In U.S. Pat No. 7,316.714, also to Gordon, the emphasis is on posterior insertion of a disc prosthesis that can provide appropriate motion. However, this latter design does not appear to accommodate for static and dynamic loading and there appears to be no accommodation for lateral and sagittal translations.
Zubok instructs in U.S. Pat. No. 6,972,038(Column 3; Line 35) that “. . . the present invention contemplates that with regard to the cervical anatomy, a device that maintains a center of rotation, moving or otherwise, within the disc space is inappropriate and fails to properly support healthy motion.” This may be true as long as translations within the prosthesis mechanism do not adequately compensate for the total motion induced by an TAR outside of the disc space. Several approaches by Ferree (U.S. Pat. Nos. 6,419,704, 6,706,068, 6,875,235, 7,048,764, 7,060,100, 7,201,774, 7,201,776, 7,235,102, 7,267,688, 7,291,171, and 7,338,525) primarily instruct how to cushion a prosthetic FSU in various ways. An exception is U.S. Pat. No. 6,706,068, which describes a design to perform certain kinematic motion of a disc prosthesis without dynamic or static cushioning support, and U.S. Pat. No. 7,338,525, which instructs on anchoring a disc prosthesis.
Aebi incorporates what essentially amounts to a hook joint (orthogonal revolute joints) in EP1572038B1 as the means for realizing motion. While the Aebi arrangement of revolute joints does allow for sagittal and lateral rotations, it does not engage in the remaining four degrees of freedom in three-space, namely, sagittal, lateral, and axial translations along with axial rotations.
Mitchell (U.S. Pat. No. 7,273,496B2) uses two revolute joints by means of orthogonal cylinders placed on top of each other and embedded as a crossbar element between vertebral plates with cavities for accepting the crossbar. This device has the limitations of motion similar to the Aebi device, and the further limitation of not linking the two plates together with the crossbar.
Khandkar (U.S. Pat. No. 6,994,727 B2) provides two orthogonal convex curvate bearing structures, with offset cylindrical radii of curvature, placed between the vertebral plates. An insert, with orthogonal, variable-curvature concave bearing surfaces, is placed between, and generally conforms to, the orthogonal convex bearings on the vertebral plates. This arrangement of bearings allows sagittal, lateral, and axial rotations of various ranges, dictated by the curvate bearing structures and the insert. The variable curvate surfaces allows some lateral and sagittal translations with FSU distractions, utilizing normal spinal forces to resist the distraction and, hence, the motion. There is no control on the forces involved, so this method could lead to possible stress on other spinal joints. The inserted device is not kinematically chained to the rest of the device and can possibly be spit out. Although, as instructed, the device is self-correcting within a limited range, tending towards a stable equilibrium established for the device in normal position. The variable curvatures can result, typically, in line-contact bearing manifolds that will wear the surfaces, possibly causing changes in the performance and characteristic motion of the device.
DiNello (US Publication No. 2006/0136062A1) instructs on how to adjust height and angulation of a motion disc after implantation.
With respect to the lower vertebra in an FSU, all possible, natural loci of motion of any four non-planar, non-collinear points located in the superior vertebra define the natural workspace of a FSU. This workspace varies from one FSU to another on the spine, creating considerable spinal disc prosthesis design problems.
The FSU workspace boundary is dictated by the sagittal, lateral and axial angle limits reported in the literature (Mow V. C. and Hayes W. C., Basic Orthopaedic Biomechanics, Lippincott-Raven Pub., N.Y., 2nd Addition, 1997). However, these angle limits do not reveal the underlying complex motion between two vertebrae in an FSU. The study by Mameren H. van, Sanches H., Beursgens J., Drukker, J., “Cervical Spine Motion in the Sagittal Plane II: Position of Segmental Averaged Instantaneous Centers of Rotation-A Cineradiographic Study”, Spine 1992, Vol. 17, No. 5, pp. 467-474 demonstrates this complexity in the cervical spine, even when the motion is restricted to flexion and extension.
In light of the above observations and limitations, it can be appreciated that there is a need for a spinal disc prosthesis that can accommodate a broader range of motions, while maintaining disc stability and integrity under static and dynamic loads.